replacing the local incomplete documentation in these Image I/O classes.The existing docs pre-date the existence of ServiceLoader which of course is further
updated in 9 to explain how to deliver service providers as modules.
Bug: http://cr.openjdk.java.net/~prr/8182610/ Webrev: http://cr.openjdk.java.net/~prr/8182610/Specdiff: http://cr.openjdk.java.net/~prr/8182610/iiospecdiff/package-summary.html
-phil
